Merge ping6 to ping
There is now a single ping binary, which chooses to use ICMP or ICMPv4
based on the -4 and -6 options, and the format of the address.
Submitted by: Ján Sučan <email@example.com>
Sponsored by: Google LLC (Google Summer of Code 2019)
MFC after: Never
Differential Revision: https://reviews.freebsd.org/D21377